Differentiation of apical, basal and mixed dendrites of fusiform cells in the cochlear nucleus

Developmental Brain Research, 56 (1990) 19-27 Elsevier

19

BRESD 51133

Differentiation of apical, basal and mixed dendrites of fusiform cells in the cochlear nucleus Laura Schweitzer Department of Anatomical Sciences and Neurobiology, University of Louisville School of Medicine, Louisville, KY 40292 (U.S.A.) (Accepted 24 April 1990) Key words: Auditory; Cell shape; Cochlear nucleus; Dendrite; Development; Morphogenesis; Polarity

Many studies suggest that the details of morphogenesis (e.g. the length and number of dendrites) are determined by factors extrinsic to the cell, while the basic form of the neuron (e.g. the shape of the soma and the placement of the primary dendritic trunks) is determined by intrinsic factors. The following study describes the development of the dendrites of fusiform cells in the dorsal cochlear nucleus of the hamster using Golgi-stained brains from hamsters of various ages. Two basic types of dendrites are described - - apical and basal - - which emanate from opposite ends of the cell body and differ in their morphology. A third type of dendrite that exits the cell laterally can create a deflection in the perimeter of the cell body altering its shape. The morphology of these dendrites is described and compared to the apical and basal dendrites. Segments of laterally extending dendrites that are near apical dendrites are qualitatively and quantitatively identical to apical dendrites (that is they branch frequently and are spine-laden) and the converse is true of the segments near basal dendrites. The results suggest that during development, whether a dendritic will be apical-like or basal-like is determined by the location of its distal segment. Thus, extrinsic factors influence the overall form of these neurons.

INTRODUCTION Although morphometric details vary from species to species, many morphologically defined cell types are easily identified across species, presumably because their overall shape is genetically determined 44. Investigations that weigh the relative roles of intrinsic and extrinsic factors in the development of neuronal form support the role of extrinsic factors in determining the details of morphogenesis 43 while suggesting that gross features can mature independent of many of the extrinsic factors normally present in the developing brain 36"4s. For example, the n u m b e r of branches on cerebellar Purkinje dendrites is correlated with the number of granule cells present at the time of branch formation s,9. Yet Purkinje cells in mutant mice lacking many afferent inputs are still easily recognizable as Purkinje cells 47"48. Thus, the basic shape of the Purkinje cells is likely to be an intrinsically regulated feature. Pyramidal cells in the cortex can be misaligned and presumably deprived of their normal pattern of input and yet develop a typical pyramidal shape and two sets of dendrites - - apical and basal - - that have different branching patterns, lengths, and dendritic appendages 64.

Perhaps the strongest evidence that basic features of neurons develop according to an intrinsic program is that cells grown away from their normal environment take on many of the characteristics that they would have developed in vivo. For example, Purkinje cells grown in vitro deprived of most of their normal inputs, develop a remarkably normal dendritic tree including spines 12,41. Pyramidal cells grown in vitro develop the morphology characteristic of pyramidal cells developing in the brain 6' 7,16,19. Thus, there is evidence that the basic morphology of neurons may be inherent. Like pyramidal cells, fusiform cells in the dorsal cochlear nucleus also have two sets of dendrites which emanate from opposite poles of the cell. Each set of dendrites has a distinct morphology - - the apical dendrites branch often and are covered with spines while the basal dendrites branch infrequently and are essentially spine-free. Fusiform cells, also known as pyramidal cells, have been found in almost every m a m m a l studied (e.g. placentals: cat1°'11'35; gerbil33; mouse52,66; rabbitlT,38; and marsupials: opossum68'69; northern native cat 2) and are easily recognized based on their position in the dorsal cochlear nucleus and their dual polarity. The incidence of similar cells across such a wide variety of species suggests

Correspondence: L. Schweitzer, Department of Anatomical Sciences and Neurobiology, University of Louisville School of Medicine, Louisville, KY 40292, U.S.A. 0165-3806/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)

20 that the basic form of the neurons is derived from a c o m m o n ancestor and therefore may be intrinsically determined. The following study describes the development of the dendrites arising from the apical and basal poles of the fusiform cells in the cochlear nucleus of the hamster. A third type of dendrite that does not exit the cell body at either pole, but rather, emanates laterally from the cell body was measured and its branching pattern analyzed. Evidence is presented that suggests that the dendritic morphology of fusiform cells is regulated by extrinsic rather than intrinsic factors.

MATERIALS AND METHODS

Animals Observations were made on the material from a collection of 6 0to 1-day-old, 3 3- to 4-day-old, 5 5-day-old, 3 8-day-old, 5 10-day-old, 18 15- to 16-day-old, 6 25- to 29-day-old, 6 45-day-old and over 50 adult (60-day-old or older) Golgi-impregnated hamster brains. These brains were prepared with a modification 59 of the Golgi-Hortega method62 developed to optimize somatic and dendritic staining in the cochlear nucleus of animals of all ages. At each age transverse, parasagittal and horizontal sections 100/~m thick through the cochlear nucleus were available for study.

Data analysis The branching pattern of individual dendrites was analyzed by the author with the Neuron Tracing System (Eutectics Inc., Raleigh, NC). Cells were chosen from the central region (midway along both the rostro-caudal and dorsomedial-ventrolateral length) of the dorsal cochlear nucleus, a region where the lamination is most distinct. Number of primary dendrites, total length of each dendrite, number of branch points per dendrite and inter-branch point distance were analyzed for dendrites in both 10-day-old and adult (postnatal day 60 or greater) hamsters. For the 10-day-old hamsters, 19 apical dendrites and 12 basal dendrites were analyzed. For the adults, 13 apical dendrites, 11 basal dendrites and 22 laterally extending dendrites were analyzed: No more than 4 dendrites were selected from any one animal. The apical dendrites are defined as dendrites that have all of their distal segments in the molecular layer of the dorsal cochlear nucleus. The basal dendrites are defined as dendrites that have all of their distal segments in the deep layer. Laterally extending dendrites are those that extend in the fusiform cell layer for a minimum distance equal to the width of the cell body. These dendrites tend to originate from the middle third of the soma. The laterally extending dendrites can be sub-categorized as being (1) apical if their distal segments are located in the molecular layer (n = 7), (2) basal if their distal segments are located in the deep layer (n = 6), and (3) mixed if their distal segments end up in both the molecular and deep layers (n = 9). The mixed dendrites have three components (1) the proximal trunk (2) the distal segments that are located in the molecular layer (referred to as the mixed-apical segments) and (3) the distal segments that are located in the deep layer (referred to as the mixed-basal segments). Dendritic branches were traced and followed from section to section when necessary. Independent 2-way analyses of variance (ANOVA) (age × dendritic type) were used to assess differences between dendritic length, number of branch points, and inter-branch point length. One-way ANOVAs were then employed to test for main effects of age (day 10 vs day 60), dendritic type at each age (apical vs basal) or dendritic subtype (apical vs laterally extending apical vs the apical portion of the laterally extending mixed; basal vs laterally extending basal vs the basal portion of the laterally extending mixed). Fisher's exact test was used to assess differences between individual means.

Statistical significance was defined as a value oi P < t).05, but trends where 0. I -- P > 0.05 were also noted and discussed wherc appropriate.

RESULTS

Polarity of the adult fusiform cell Fusiform cells in the dorsal cochlear nucleus (DCN) of the adult hamster have two sets of dendrites that differ morphologically from one another (Fig. 1). The dendrites emanate from two poles of the cell, the distal processes are located in two different regions of the nucleus, and their appearance is quite distinct. The fusiform cells are, for the most part, bipolar in appearance with a superficial pole called the apical pole and a deep pole called the basal pole. The n u m b e r of apical and basal dendrites is about the same (Fig. 2). For example, in the adult, there is an average of 2.6 ( + 0,24) apical dendrites and 2.2 ( + 0.49) basal dendrites. While the number is similar, the morphology of the dendrites originating at each pole is very different. In the adult, individual apical dendrites are over two times longer than individual basal dendrites (Fig. 3, solid bars). The apical dendrites branch repeatedly, that is they have shorter branches than the basal dendrites (Fig. 4, solid bars) and three times as many branches as the basal dendrites (Fig. 5, solid bars). Apart from their unique branching patterns, the appearance of the dendrites differs since the apical dendrites are covered with dendritic spines, while the basal dendrites are smooth, and almost entirely devoid of appendages (Fig. 1). The distal processes of the two morphologically distinct sets of dendrites are located in two different regions of the dorsal cochlear nucleus (Fig. 1). The cell bodies of the fusiform cells form the fusiform cell layer which ties deep to a soma-sparse layer called the molecular layer. The fusiform cell layer lies superficial to another somasparse area, which is part of the deep layer. Thus, the fusiform cell layer is surrounded on both sides by regions where the neuropil is composed primarily of axons and dendrites. The apical dendrites extend toward the pial surface of the nucleus, into the molecular layer, and the basal dendrites extend toward the restiform body, into the deep layer. The cell bodies of the fusiform cells in the hamster are often not bipolar, fusiform, or spindle-shape in appearance. They are sometimes pyramidal, spherical, or even multipolar in shape (Fig. 1). Major dendrites often exit the cell body laterally, rather than apically or basally, causing lateral deformation of the cell's perimeter. Sometimes these laterally extending dendrites turn immediately and join the apical or basal dendritic trees (Fig. 1C,D). Occasionally, the dendrites spread a long distance

21 laterally in the fusiform cell layer before turning up or down to join the apical or basal tree. In these cases the laterally extending dendrites branch very rarely while in the fusiform cell layer; once they cross to the molecular or deep layer these dendrites acquire the morphology of the apical or basal dendrites, respectively. Using the p a r a m e t e r s measured here, the laterally directed dendrites that reach the molecular layer are identical to other apical dendrites (Figs. 4 and 5, compare solid bars to open bars), that is they branch often, have short branch lengths, and are covered with spines like other apical dendrites. Similarly, without exception, the laterally directed dendrites that turn toward the deep layer branch infrequently, have long branch lengths and have few

appendages, like other basal dendrites (Figs. 4 and 5, compare solid bars to open bars). A third type of dendrite that exits the soma laterally cannot be classified as apical or basal (Fig. 1A,B). These dendrites branch infrequently in the fusiform cell layer but send branches both up into the molecular layer and down into the deep layer. These dendrites will be referred to here as 'laterally extending mixed dendrites'. The distal processes of these dendrites acquire the morphology of the dendrites adjacent to them (Figs. 4 and 5, checkered bars). Thus, the processes that end up in the molecular layer branch often, have short branches and have spines all over them like apical dendrites. On that same dendrite, processes that end up in the deep

ml •

fcl

cl

.,.

ml

Fig. 1. Golgi-impregnated fusiform cells from the adult hamster. The cell bodies of these cells are not fusiform-shaped and are more accurately described as multipolar. In addition to the apical dendrites in the molecular layer (ml) and the basal dendrites in the deep layer (dl) each of the cells shown have a mixed, laterally extending dendrite. A,B: the proximal portion of the laterally extending dendrite (arrow) exits the cell laterally, courses in the fusiform cell layer (fcl), and terminates in both the molecular layer and deep layer. The portion of the dendrite labelled 'a' joins the apical tree while the portion labelled 'b' joins the basal tree. C: the laterally extending dendrite on this cell (a) is classified as an apical dendrite, since it only ends in the molecular layer. D: photomicrograph of cell drawn in C. Scale = 20/~m.

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Number of neurites/primary dendrites i

Branch Length

r

[]apical

-7-

7

120

m,asal

IO0

BO

K_ r-

Z

e

6O 4O

2

2O 0

day 5

day 10

day 60+

age (days}

age (days}

Development of polarity

Fig. 4. Length of individual second- and higher-order branches on apical and basal dendrites. This analysis was done in order to relate how the distal dendrites compared. Since the laterally extending primary trunks are very long and confound this data, they have been eliminated from this analysis. The apical dendrites have shorter branches than the basal dendrites (F = 9.98; df = 1,53; P < 0.01) and this effect is apparent by day 10 (F = 5.80; df = 1,22; P < 0.03). Branch length stays stable between postnatal day 10 (striped bars) and adulthood (solid bars). The laterally extending dendrites (open bar) and laterally extending mixed dendrites :(checkered bar) have the same branch lengths as the adjacent non-laterally extending dendrites (solid bars).

A t birth the fusiform cells in the dorsal cochlear n u c l e u s of the h a m s t e r are n o t spatially segregated into a layer as t h e y will b e in the adult (Fig. 6). T h e cell b o d i e s are l o c a t e d in a single mass of large cells. A t this age the cells are v e r y i m m a t u r e , with slim n e u r i t e s e x t e n d i n g

place except for a t h i c k e n i n g of the m o s t p r o x i m a l p o r t i o n s of s o m e of t h e p r i m a r y d e n d r i t e s . T h e cells are still essentially r a d i a t e in a p p e a r a n c e . I n t e r e s t i n g l y , the n u m b e r of n e u r i t e s in n e o n a t e s is similar to the n u m b e r

Fig. 2. The number of neurites (postnatal day 5) and the number of dendrites (older ages) on fusiform cells. The number of neurites includes both neurites that will be apical and basal dendrites and an axon since these cannot be differentiated in the Golgi material at this age. No differences between ages were found (all P's > 0.10). layer b r a n c h i n f r e q u e n t l y , h a v e l o n g b r a n c h e s , a n d are s p i n e - f r e e like basal d e n d r i t e s .

f r o m the cell b o d y in all directions. R a t h e r t h a n b e i n g b i p o l a r in a p p e a r a n c e , the cells are r a d i a t e in form at this time. B e t w e e n birth a n d day 5 the cells r e m a i n very i m m a t u r e in a p p e a r a n c e . E s s e n t i a l l y n o c h a n g e takes

of c o m b i n e d apical a n d basal d e n d r i t e s in the adult cells (Fig. 2). A t day 5 the cells in t h e mass of large cells a r e still n o t segregated i n t o layers. A s u b t l e t r a n s f o r m a t i o n takes

Branch Points on Individual Dendrites 18

Tapical Dendritic Length 800

basal

I

c -,--i

g

600

c

~

9

"g

B

"E

3

E

c-iJ

400

C

200

0

age (days} age (days} Fig. 3. The length of individual apical and basaldendriteson day 10 and 60. Continued growth occurs for the apical (F = 18.28, df = 1,30; P < 0.001) and the basal (F = 4.65; df = 1,21; P < 0.05) dendrites between days 10 and 60. On day 10 the apical dendrites tend to be longer than the basal dendrites although these differences do not reach significance (F = 3.06; df = 1,29; P = 0.09). In the adult, apical dendrites are longer than basal dendrites (F = 4.53; df = 1,22; P < 0.05).

Fig. 5. Number of branch points on individual apical and basal dendrites. Branches are added onto apical (F = 13.14; df = 1,30; P < 0.01) but not basal (F = 2.10; df = 1,21; P > 0.10) dendrites between days 10 (striped bars) and adulthood (solid bars). There are more branch points on apical dendrites on day 10 than on basal dendrites at this age (F = 8.42; df = 1,29; P < 0.1). There are also more branch points on apical dendrites than basal dendrites in the adult (F = 6.97; df = 1,22; P < 0.02). There are no differences between the numbers of branch points on laterally extending (open bars), laterally extending mixed (checkered bars) and their adjacent non-laterally extending counterparts (solid bars).

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W--C

v

Fig. 6. Drawing of several Golgi-impregnated large cells in the dorsal cochlear nucleus of the neonatal hamster. Scale = 50 pm. Transverse section, d, dorsal; m, medial; 1, lateral; v, ventral.

place in those cells located most superficially in the mass. These presumptive fusiform cells have taken on a bipolar appearance, with their long axis perpendicular to the surface of the DCN (Fig. 7). That is, dendrites still emanate in all directions from the cell body, but the dendrites that are near to the molecular layer seem to be preferentially directed superficially into that layer. To a lesser extent, deeper dendrites are directed away from the molecular layer. Some limited branching has occurred. Many cells that are deeper in the mass of large cells retain the radiate appearance of the immature cells. By day 10, the fusiform cells are segregated from the large cells (presumptive giant cells) in the deep layer by

Fig. 8. Drawing of several Golgi-impregnated large cells in the dorsal cochlear nucleus of the 10-day-old hamster. Scale = 50/~m. Transverse section, d, dorsal; m, medial; 1, lateral; v, ventral; stars, fusiform cells.

a small space that is relatively soma-flee, and therefore a true fusiform cell layer can be identified (Fig. 8). Many of the cell bodies in the fusiform cell layer are more spindle shaped with dendrites that arise from about the upper third of the cell aimed upward into the molecular layer and dendrites from about the lower third of the cell body aimed deep. These two sets of dendrites have many of the features of their adult counterparts: the number of primary dendrites that can be classified as apical and basal dendrites is similar to the adult (Fig. 2), the apical dendrites already have twice the number of branches as the basal (Fig. 5, striped bars), and the apical dendrites branch repeatedly and have shorter branch lengths (Fig. 4, striped bars) than the basal dendrites. Although the lengths of branches of the apical and basal dendrites differ from each other at each age, the apical branch length remains stable between day 10 and 60 (Fig. 4, compare striped and solid bars). Similarly the length of the branches of basal dendrites remains the same. Thus, the length of branches can be considered a stable characteristic of each type of dendrite regardless of developmental stage. Total dendritic length increases (Fig. 3) and the spines on the apical dendrites mature after day 10, but essentially all of the features that define the dual polarity of the fusiform cells are mature at this age.

d C

--r

DISCUSSION

Branching patterns

Fig. 7. Drawing of several Golgi-impregnated large cells in the dorsal cochlear nucleus of the 5-day-old hamster. Scale = 50 pm. Parasagittal section, d, dorsal; v, ventral; c, caudal; r, rostral; stars, presumptive fusiform cells.

Two distinct patterns of dendritic morphology are found for the apical and basal dendritic trees of the fusiform cells in the dorsal cochlear nucleus of the hamster. The apical dendrites branch often, have short inter-branch point lengths, and are covered with spines. In contrast, the basal dendrites branch rarely, have long

24 branch lengths, and are smooth in profile. These two types of dendrites are located in two different layers with two distinct sets of inputs. The apical dendrites are located in the molecular layer which contains the axons of the granule cells24'3~'32 and inputs from the anteroventral cochlear nucleus 23. The vast majority of the GABAergic and glycinergic input is located in this layer5'3°'5°'67. The basal dendrites are located in the deep layer, the location of inputs from the cochlear nerve 24'26"55, superior olivary nuclei 23"25, inferior colliculus 14, periolivary nuclei and the nucleus of the trapezoid body 61. Different inputs may play a role in determining the morphology of the two sets of fusiform cell dendrites. The timing of the development of some inputs coincides with the development of the fusiform cell dendrites they contact. While the development of the descending inputs from the inferior colliculus, superior olive, and medial nucleus of the trapezoid body to the dorsal cochlear nucleus have not been studied, two major inputs to the fusiform cells have been investigated. Ingrowth and initiation of synapse formation of the cochlear nerve fibers occurs during the first two postnatal weeks in the hamster as demonstrated by the rapid-Golgi method for impregnating axons in immature animals and electron microscopy58. These events coincide with outgrowth of the basal dendrites, a target of the cochlear nerve fibers 24. Although a few terminals that resemble granule cell terminals can be seen as early as three days after birth 5s, the granule cells, which provide input to the apical dendrites 3~, mature and migrate to their adult location over a prolonged postnatal period 57. Continued development of the apical dendrites occurs during this extended period, especially the formation of spines (discussed below). In the adult, repetitive branching occurs, for the apical dendrites, at the point that these dendrites enter the molecular layer. The apical dendrites have their unique attributes only beyond the point at which they enter the molecular layer. Similarly the features with which we characterize basal dendrites are confined to dendrites in the deep layer. Laterally extending dendrites become part of the apical or basal dendritic tree, with features common to the tree, depending on the location of their terminal branches. The proximal portions that course through the fusiform cell layer are virtually branchless. Branching occurs just as these dendrites turn into the adjacent synaptic zones. Perhaps the most supportive evidence for the important role of the environment on dendritic morphology can be found in the mixed laterally extending dendrites that become both apical and basal dendrites. While it is

possible that signals originating in the cell body could be transported down a common dendritic trunk and direct differential maturation of the apical and basal dendrites, it seems more likely that the environment surrounding these disparate distal sections determines their dendritic morphology. There is considerable evidence that the pattern of dendritic branching is influenced by the afferents to the developing dendrites (reviewed in Rakic 46 and for the auditory system37"51). Extra innervation provided by transplanted inputs will increase branching of neurons contacted by the inputs TM. In the cerebellum a linear relationship exists between the number of granule cells in cerebellar cortex at the time of Purkinje cell morphogenesis and the number of dendritic branches that form on the Purkinje cells9. For Purkinje cells, inter-branch point length is inversely related to the number of branch points and is, therefore, also correlated with the afferent inputs present during development s. Thus, afferents regulate dendritic branch formation and inter-branch point length. Although it is not known how afferents influence dendritic development, recent evidence suggests that deafferentation decreases the density of cytosketetal elements (microtubules and neurofilaments) in the postsynaptic dendrite 15. Conversely, afferents and other cellular contacts may reduce membrane fluidity and thus stabilize the cytoskeletal elements within developing neurites 4°. While the initial interaction of the afferent with the neurite is with the cell surface, membrane contact by afferents may, in turn, alter the association of anchoring proteins (ankyrin, spectrin and/or fodrin) with the cytoskeletal elements on the cytoplasmic side of the membrane 39. Thus, it seems plausible that afferents can indirectly increase cytoskeletal stabilization during neuritic development.

Dendritic length Another characteristic that differentiates apical from basal dendrites is dendritic length. Growth may be dependent on the presence of afferents as dendritic growth may largely be a result of the addition of new dendritic branches le. Dendritic growth of the apical dendrites of fusiform cells is due to the addition of new branches 56. It has been demonstrated that growth of subpopulations of dendrites on individual cells via the addition of dendritic branches occurs in regions where synaptogenesis is most active 65. Thus, three features that differentiate the basal from the apical dendrites: (1) numbers of branches, (2) branch length, and (3) total dendritic length are known to be influenced by afferent input. It seems likely that the disparate inputs in the two layers of the DCN control the morphology of the two sets of dendrites.

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Spine formation Another attribute that differentiates the basal from the apical dendrites is the absence or presence, respectively, of spines. Spine formation and maintenance is an intrinsic property for some cells and an extrinsically induced property of other cells. Spine formation on Purkinje cells in the cerebellum seems to occur independent of input. Spines develop on Purkinje cells in cerebella severely depleted of their granule cell inputs 3'22'47'49 and these spines can persist in the absence of the presynaptic inputs 27. Spines are a feature of Purkinje cells grown in culture 13"41'42'6°. In contrast, spine formation and maintenance is dependent on intact inputs in the hippocampus and cerebral cortex 18'53, In the case of the fusiform cells, the presence of spines seems to be dependent on the inputs to the dendrites. Spines are contacted by terminals from small unmyelinated axons, presumably the axons of the granule cells 31. Spine formation coincides with the prolonged period during which the granule cells are continuing to migrate into their adult location 57. The location of granule cell axons, in the molecular layer, correlates with the location of the spines on the apical dendrites. Dendrites crossing to the molecular layer become spineladen. Laterally extending dendrites will have spinecovered processes located in the molecular layer and spine-free processes located in the deep layer.

Polarity of the fusiform cells While there is considerable evidence that the details of dendritic morphology are determined by extrinsic influences, the basic form of a neuron (the soma shape and where the major dendrites will be located) often seems to develop normally even in the most extraordinary circumstances. In many cases there seems to be a primary influence that determines cell shape and the initial location of neurites, and another, secondary influence that refines dendritic branching patterns 54. The first influence is thought to be intrinsic to the cell and unalterable, while the second is thought to be extrinsic and malleable 46. Descriptive observations are presented here that suggest the basic form of some neurons may be determined by the inputs surrounding them. The overall shape of the fusiform cells in the dorsal cochlear nucleus of the hamster is related to how the dendritic trunks exit the cell body. For the most part, the shape is that of a bipolar neuron with two sets of dendrites. The apical dendrites exit the cell body and aim upward into the molecular layer; the basal dendrites aim downward. The cells are not, however, truly bipolar since single dendrites can be both apical and basal. The cell bodies can adopt a multitude of forms depending on how the dendrites grow.

When dendrites exit the cell body laterally and remain in the fusiform cell layer before turning down into the deep layer, the cell body appears pyramidal. When more than one dendrite does this, the cell body has a multipolar appearance. Inverted pyramids are formed when laterally extending dendrites turn upwards into the molecular layer. Thus, even the basic form of the neuron may be influenced by dendritic growth. Definitive evidence for this influence might be provided by growing these cells with select subpopulations of their normal inputs, either by deafferentation or in vitro. Fusiform cells in the hamster and their homologues in other species have a variety of shapes that may be dependent on their developmental environment. At one extreme large cells in the dorsal cochlear nucleus of the mouse called 'Purkinje-like cells '66 are located in the fusiform cell layer but only have dendrites extending into the molecular layer. Although the dendrites are indistinguishable from the apical dendrites of fusiform cells, the cell bodies of these cells are not bipolar but rather are spherical. Conversely, giant cells only have dendrites in the deep layer of the DCN. These large cells have a radiate appearance and have dendrites that branch infrequently and lack spines. As noted in Schweitzer and Cant 59 the giant cells and fusiform ceils arise at the same time and from the same proliferative zone 4'28'34'63 migrate together 69 and remain together in one mass until well after birth 59. Perhaps the differentiation of the two cell types is determined by the neuropil they come into contact with during development. Fusiform cells can be identified in many mammals with the notable exception of humans. In humans, multipolar cells with long dendrites that branch infrequently and have few appendages occupy the DCN in the position ordinarily reserved for fusiform cells 1. Moore and Osen 29 suggest that the large cells found in man might be modified fusiform cells. In contrast, Adams 1 suggests that these cells may simply be giant cell homologs like those found in the deep layers of the DCN in non-human mammals. It is interesting to note that humans lack components found in the molecular layer of the other mammals including granule cells and their axons 21. If these components must be present for the formation of dendrites with the characteristics of apical dendrites, apical-like dendrites would not develop in the human. The form of the cells in the adult human DCN, like those in non-human mammals, may simply be a product of their developmental milieu.

Acknowledgements. The author would like to thank Drs. Nell B. Cant, Kenneth Leskawa and John Wible for their comments and criticisms and Ms. Tina Cecil for her technical assistance. This work was supported by National Institutes of Health Grant DC-00233.

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